It is known that conventional crystalline polypropylene is useful for producing various shaped articles which must exhibit excellent physical properties, such as rigidity and resistance to thermal deformation. However, it is also known that the conventional crystalline polypropylene exhibits a poor lacquer-bonding property and a poor resistance to mechanical impact applied thereto because of non-polarity and high crystallinity of the polymeric molecules thereof, and has unsatisfactory physical properties at a low temperature of, for instance, -30.degree. C. or lower. Therefore, the conventional crystalline polypropylene is not adequate as a material for producing shaped articles which simultaneously satisfy all the properties of excellent resistance to mechanical impact, lacquer-bonding property, regidity and resistance to thermal deformation.
In order to enhance the resistance to mechanical impact and to enhance the lacquer-bonding property of the crystalline polypropylene, it was attempted to blend the conventional crystalline ethylene-propylene block copolymer with an amorphous ethylene-propylene copolymer alone or a mixture of the amorphous ethylene-propylene copolymer with various types of polyethylene.
However, in this attempt, it was found that the above-mentioned modes of blends caused the resultant copolymer blends to exhibit lower rigidity and resistance to thermal deformation than those of the crystalline ethylene-propylene block copolymers.
Japanese Patent Application Laid-open No. 53-64257 (1978) discloses an approach for producing a high impact polypropylene composition having an improved resistance to mechanical impact. The composition comprises a crystalline ethylene-propylene block copolymer, amorphous ethylene-.alpha.-olefin copolymer and talc. The above-mentioned publication states that the composition exhibits a superior resistance to mechanical impact at a low temperature, regidity and lacquer-bonding property when compared to those of the conventional crystalline ethylene-propylene block copolymers, and describes certain working examples of the composition.
It was found by the inventors of the present invention that though the rigidity, resistance to thermal deformation and resistance to mechanical impact of the composition desclosed in the above-mentioned Japanese publication are satisfactory, the lacquer-bonding of the composition is unsatisfactory. Also, it was found that the surface of shaped articles made from the composition of the above-mentioned Japanese publication exhibits an unsatisfactory gloss and, sometimes undesirable flow marks are formed thereon during the shaping procedure. Therefore, the composition disclosed in the above-mentioned Japanese publication is usable only for shaped articles which are not strictly required to have an excellent lacquer-bonding property and a satisfactory luster (gloss). In other words, the composition disclosed in the Japanese publication is not adequate for producing various parts of motor vehicles, for example, bumpers, which must exhibit an excellent appearance, in addition to having excellent mechanical and physical properties.
Under these circumstances, it was strongly desired to provide a new type of polypropylene composition capable of producing shaped articles having all of the following properties: excellent Izot impact strength at a low temperature; excellent rigidity and resistance to mechanical impact, from the standpoint of rigidity; an improved lacquer-bonding property; and a superior appearance.